172 PRACTICAL BACTERIOLOGY. 



On the second day small punctiform colonies ap- 

 pear, which gradually become rounded and golden- 

 yellow, increasing in size until they are as large as 

 pins' heads. As soon as they reach the surface of the 

 gelatine, the latter immediately begins to liquefy, 

 although slowly. Around the colony a very shallow 

 depression, several millimetres in breadth, is formed ; 

 the border of this depression stands out sharply from 

 the surrounding gelatine, although it is only to be 

 plainly distinguished in exactly the right light. This 

 shallow depression is caused by the liquefaction of the 

 gelatine. By degrees the colonies increase in size 

 until finally they coalesce and the whole of the gelatine 

 is liquefied. 



If, before the colonies run together, a portion of 

 one is conveyed into some liquid agar-agar, and then 

 some of the mixture is poured out on to a plate, the 

 behaviour of the Staphylococcus pyogenes aureus at 

 higher temperatures may be observed. At blood heat 

 it grows somewhat more rapidly, and, since agar-agar 

 is never liquefied by bacteria, and therefore it is no 

 longer to be feared that the colonies will break down 

 and coalesce, they may be observed for a much longer 

 time, only of course the agar-agar muse be kept moist. 

 Its behaviour in puncture cultivations is character- 

 istic. At first a golden yellow film, which sinks in 

 somewhat, appears on the surface of the gelatine, 

 whilst along the track of the needle there is only a 



